Conventional collapsible mechanism at the top of a collapsible tent, for example disclosed are in the Chinese patent database with patent number ZL02220066.5, in the American patent database with U.S. Pat. No. 7,861,736, U.S. Pat. No. 7,607,447 and U.S. Pat. No. 8,047,218. The conventional mechanism generally comprises a central base, a lower pivot base, a sub-pole, a supporting pole and a spring. The central base comprises a vertical pole and an upper pivot base fixed to the upper end of the vertical pole, the lower pivot base is slidably connected to the vertical pole; the spring is sleeved on the vertical pole and abuts against between the upper pivot base and the lower pivot base; the lower end of the sub-pole is pivot joint with the lower pivot base, the upper end of the sub-pole is pivot joint with the supporting pole, the upper end of the supporting pole is pivot joint with the upper pivot base. When it is collapsed, one structure is that the lower pivot base slides upwards; another structure is that the lower pivot base slides downwards. Therefore, the collapsible mechanism has disadvantages as below: the vertical pole has to be long enough to make sure that the sub-pole and the supporting pole are folded in parallel when it is collapsed, thus reducing inner space of the tent, and the appearance of the tent is not attractive.